Stream

Remarks#

Streams are composable, lazy enumerables.

Due to their laziness, streams are useful when working with large (or even infinite) collections. When chaining many operations with Enum, intermediate lists are created, while Stream creates a recipe of computations that are executed at a later moment.

Chaining multiple operations

Stream is especially useful when you want to run multiple operations on a collection. This is because Stream is lazy and only does one iteration (whereas Enum would do multiple iterations, for example).

numbers = 1..100
|> Stream.map(fn(x) -> x * 2 end)
|> Stream.filter(fn(x) -> rem(x, 2) == 0 end)
|> Stream.take_every(3)
|> Enum.to_list

[2, 8, 14, 20, 26, 32, 38, 44, 50, 56, 62, 68, 74, 80, 86, 92, 98, 104, 110,
 116, 122, 128, 134, 140, 146, 152, 158, 164, 170, 176, 182, 188, 194, 200]

Here, we chained 3 operations (map, filter and take_every), but the final iteration was only done after Enum.to_list was called.

What Stream does internally, is that it waits until actual evaluation is required. Before that, it creates a list of all the functions, but once evaluation is needed, it does goes through the collection once, running all the functions on every item. This makes it more efficient than Enum, which in this case would do 3 iterations, for example.